1. Field of the Invention
The present invention relates to a fault current detection device for discriminating a short-circuit occurred in a d.c. network such as d.c. feeder or trolley of a d.c. electric railway.
2. Description of the Prior Art
A load current flowing through a d.c. feeder of a d.c. electric railway tends to be substantially increased recently with frequency train service and in the number of cars in a train. For this reason, short-circuit current also tends to increase, but the ratio of increase thereof is smaller than that of the load currents. It becomes very important to protect electric cars and/or system devices against short-circuit of the d.c. feeder and/or trolley wire.
A unit length of the d.c. feeder is generally from several to ten kilometers or more. With increase in the capacity of d.c. substation, a peak value of fault current due to a distant short-circuit may be lower than a pick-up setting value of a d.c. high speed circuit breaker for d.c. feeder protection. On the other hand, when the pick-up setting value is selected in a manner that such fault current is reliably cut-off, a maximum value of current to be fed is in inverse-proportion to the length of the feeder and becomes substantially small when the length is large. Therefore, in order to detect a short-circuit fault caused in the d.c. network as early as possible and to reliably cut-off a fault current caused thereby, a selective protecting system in which the fault current is discriminated on the basis of a difference in circuit phenomena between a load current and a short-circuit current and tripped selectively, has been employed.
An example of such a selective trip device is one which utilizes the fact that a rising rate of a usual load current is relatively small and a magnitude of variation thereof is small, while a fault current rises at a higher rate and the variation thereof is large. In such a device, a variable .DELTA.I.sub.o representative of an amount of current variation caused within a predetermined time constant t.sub.d is monitored continuously and a fault current detection signal is provided when the variable .DELTA.I.sub.o exceeds a set value .DELTA.I.sub.r. In this selective trip device, a variation of a d.c. current I flowing through the d.c. feeder or trolley wire is detected through a current transformer. The detected variation is supplied to an integrator composed of a parallel circuit of an integrating resistor and an integrating capacitor connected in series with the resistor and an output resistor, a current .DELTA.I.sub.o flowing through the latter being used to detect a fault.
In such a circuit, a tranfer function G(S) between the current I and the current .DELTA.I.sub.o can be represented by the following equation (1): ##EQU1## where C is a capacitance of the integrating capacitor and R is a resistance of the integrating resistor and the output resistor.
In order to put a circuit phenomenon of the integrating circuit in a primary side of the current transformer, M=-2Rt.sub.d is used in the equation (1). Thus, the transfer function G(S) can be modified as follows: ##EQU2##
There is a sampling method using a microcomputer as an example of a conventional system operable according to this principle, in which the transfer function represented by the equation (2), is approximated by the following equation (3). EQU .DELTA.I.sub.n =(I.sub.n -J.sub.n-1)(1-.DELTA.t/t.sub.d) (3) EQU J.sub.n =J.sub.n-1 +.DELTA.I.sub.n .DELTA.t/t.sub.d ( 4)
where .DELTA.I.sub.n is a variable representing an amount of current variation at nth sampling time, I.sub.n is an instantaneous value of current at nth sampling time, J.sub.n is a final value of current weighted at nth sampling time, J.sub.n-1 is an initial value of current weighted at nth sampling time, .DELTA.t is a sampling period and t.sub.d is a time constant of the integrating circuit.
In this sampling method, the variable .DELTA.I.sub.n representing an amount of current variation occurred within a time period shorter than the time constant t.sub.d is monitored continuously and a detected signal is provided when the variable .DELTA.I.sub.n exceeds a set value .DELTA.I.sub.r. That is, the following equation (5) is a discrimination equation upon which a fault is selectively determined. EQU (I.sub.n -J.sub.n-1)(1-.DELTA.t/t.sub.d)&gt;.DELTA.I.sub.r ( 5)
On the other hand, another system is also proposed which utilizes not the d.c. current I but a variation thereof, D=dI/dt. When this variation is used, the transfer function G(S) between the variation D and the current .DELTA.I.sub.o is as follows: ##EQU3##
When, in the equation (6), M=-2Rt.sub.d to put the circuit phenomena of the integration circuit in the primary side of the current transformer, the equation (6) can be modified as follows: EQU G(S)=.DELTA.I.sub.o (S)/D(S)) (7)
In the sampling method using a microcomputer operating according to the equation (7), the latter is approximated by the following equation (8). EQU .DELTA.I.sub.n =.DELTA.I.sub.n-1 (1-.DELTA.t/t.sub.d)+D.sub.n..DELTA.t (8)
where .DELTA.I.sub.n is a variable representing an amount of current variation an nth sampling time, .DELTA.I.sub.n-1 is a variable representing an amount of current variation at (n-1)th sampling time, D.sub.n is an instantaneous value of current changing rate at nth sampling time, .DELTA.t is a sampling period and t.sub.d is a time constant of the integrating circuit.
In this sampling method, the variable .DELTA.I.sub.n which represents the amount of current variation occurred within a time period shorter than the time constant t.sub.d is monitored continuously as in the former case and a detection signal is provided when the variable .DELTA.I.sub.n exceeds a set value .DELTA.I.sub.r. That is, a fault is selectively determined according to the following equation (9): EQU .DELTA.I.sub.n-1 (1-.DELTA.t/t.sub.d)+D.sub.n..DELTA.t&gt;.DELTA.I.sub.r ( 9)
It should be noted, however, that the variable .DELTA.I.sub.n in the equation (3) or (8) is influenced by initial conditions. That is, when a load current reduces abruptly immediately before a short-circuit occurs in a certain feeder section, the variable .DELTA.I.sub.n may become smaller than that under the initial conditions which are zero. Therefore, under the most undesirable circumstance where a short-circuit is caused immediately after the load current reduces abruptly, the fault tends to be not detected due to the fact that the variable .DELTA.I.sub.n does not reach the set value.
In order to avoid such a situation, it has been usual to weight the variable .DELTA.I.sub.n with respect to the load current within a preceding specified time to obtain a collective parameter as the initial condition. For this reason, it is impossible to reflect faithfully a variation of load current immediately before the short-circuit. Thus, it becomes impossible to detect the fault current when the train running becomes frequent and a variation of load current becomes considerable.